On the Architecture of Beetle Elytra 191
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On the architecture of beetle elytra 191 Entomologie heute 22 (2010): 191-204 On the Architecture of Beetle Elytra Zur Architektur von Käferelytren THOMAS VAN DE KAMP & HARTMUT GREVEN Summary: We examined the elytron cuticle of various beetle species histologically and by scanning electron microscopy (SEM). All cuticles examined appear to be of pseudorthogonal type, i.e., single orthogonal layers of microfibrils are delineated by intervening helicoidal microfibrils. In the endocuticle two “types” of “lamellae” (i.e. cuticular layers visible in histological sections) can be distinguished. One type is characterised by successive plies of unidirectional arranged microfibrils, in the other the plies are subdivided in paralleling rods consisting of thick bundles (macrofibres) of microfibrils that are unidirectional oriented along the bundle. Transitional stages between the two types are expected to occur. Successive plies and successive bundles cross at various angles. The “type” with the macrofibres corresponds to the “balken cuticle” of the classical German literature. Further, data from the elytra of 40 beetle species of 24 families point to the considerable variation regarding the total thickness of an elytron, the thickness of the dorsal and ventral cuticle of an elytron and the width of the hemolymph space within elytra. Key words: insect cuticle, elytra, “balken”, pseudorthogonal cuticle Zusammenfassung: Wir haben die Elytrencuticula verschiedener Käferarten histologisch und mit Hilfe des Rasterelektronenmikroskops (REM) analysiert. Wir gehen davon aus, dass alle untersuchten Cuticulae vom pseudorthogonalen Typ sind, d. h., dass die einzelnen orthogonalen Lagen aus Mikrofibrillen von helicoidal angeordneten Mikrofibrillen begrenzt werden. In der Endocuticula sind zwei „Typen“ von „Lamellen“ (einzelne Lagen der Cuticula im histologischen Schnitt) zu unterscheiden, die wahrscheinlich durch Übergänge miteinander verbunden sind. Ein „Typ“ ist durch Lagen mehr oder weniger flächiger Schichten einheitlich ausgerichteter Mikrofibrillen charakterisiert, im zweiten „Typ“ bestehen diese Lagen aus parallel liegenden, weitgehend getrennten, dicken Bündeln (Makrofibrillen) von Mikrofibrillen, die einheitlich längs des Bündels ausgerichtet sind. Aufeinander gestapelte Lagen und Bündel kreuzen sich unter verschiedenen Winkeln. Der „Typ“ mit den parallelen Makrofibrillen entspricht der „Balkencuticula“ der klassischen deutschsprachigen Literatur. Zudem belegen wir bei 40 Käferarten aus 24 Familien eine beträchtliche Variation in der Dicke der gesamten Elytre, auffallende Unterschiede in der Dicke der Cuticula der ventralen und dorsalen Cuticula sowie der Ausdehnung des Hämolymphraums zwischen beiden. Schlüsselwörter: Insektencuticula, Elytren, Balken, pseudorthogonale Cuticula 1. Introduction multilayered, being divided into an outermost epicuticle preceded by the chitin containing The insect cuticle is a multifunctional procuticle, in which a dense exo- (= preecdysal lightweight composite material, which has procuticle) and a thicker endocuticle may be over the years attracted much attention in distinguished. This holds also for the elytra, various fields (summarized e.g. by RICHARDS fore wings in the Coleoptera that cover the 1951; NEVILLE 1975, 1993, 1998; VINCENT & membranous hind wings, when the latter are WEGST 2004). As seen throughout the folded up at rest. Elytra develop from the arthropods, the adult insect cuticle is imaginal discs and are principally evaginations Entomologie heute 22 (2010) 192 THOMAS VAN DE KAMP & HARTMUT GREVEN of the epidermis. Therefore they exhibit an When checking the literature, we became aware outer (dorsal) and an inner (ventral) cuticle, of two “types” of pseudorthogonal cuticles. which enclose the haemolymph space One “type” has successive layers (“plies”) traversed by trabeculae, which connect upper made up of microfibrils with a unidirectional and lower elytral surfaces. Elytra of beetles orientation (for the first time described in are “hard” and highly sklerotised (see Tenebrio molitor: NEVILLE & LUKE 1969; textbooks of entomology). NEVILLE 1975). The other “type” shows Exo- and endocuticle often show unit layers parallel bundles (macrofibres) of tightly termed “lamellae” when viewed under the packed microfibrils orientated unidirectional light microscope, but more so when studied along the bundle, which are separated from with the transmission electron microscope. each other by a small gap, or macrofibres may In sections through the cuticle of insects, form a network (see below). Macrofibres parabolic arcs (BOULIGAND 1965) or spiral change their direction in successive layers (e.g. patterns of microfibrils (e.g. MEYER-ROCHOW Pachnoda marginata, HEPBURN 1972; HEPBURN 1975) are frequently visible under both LM & BALL 1973; DENNELL 1976; GREVEN & and TEM, but as explained in detail by SCHWINGER 2005). The bundles of BOULIGAND (1972) and others, these patterns macrofibres correspond to the “balken” are optical artifacts. The cuticle consists of described in the classical German literature laminae of chitin microfibrils that run parallel (summarized in RICHARDS 1951). MEYER to the cuticle surface and are embedded in a (1842; fide BIEDERMANN 1903) was the first, protein matrix. Within each lamina the who found that each “lamella” in the cuticle microfibrils are oriented in the same direction, of Lucanus cervus was composed of parallel but in successive laminae there is a slight and partly anastomosing “clear rods” (“glas- rotation in orientation, and for every 180o a helle Stäbchen”) and that successive lamellae lamella is observed in sections normal to the crossed at various angles. In Oryctes nasicornis surface. these rods, however, form a network (see also In cuticles of other species, however, KAPZOV 1911). The term “balken” or “balken- unidirectional oriented microfibrils form a lage” was introduced by KAPZOV (1911), who preferred layer (“ply”), which represents noticed that “balken” might fuse to form a strictly speaking a thick lamina. Most of more or less continuous “lamella” (see also them do not consist exclusively of uni- KÜHNELT 1928). directional “plies”, i.e. they are not strictly In later articles, authors did not clearly orthogonal, but “plies” exhibit a thin differentiate between continuous plies and intervening layer of helicoids. Therefore, “balken”. STEGEMANN (1930: p. 11) wrote that these systems have been called pseud- the “balkenlage” of the elytra in Cicindelae orthogonal (see the two-system model for (sic!) consisted of clear plates with parallel chitin-protein complexes in insect cuticles fibrils and similarly SPRUNG (1932: p. 447) by NEVILLE & LUKE 1969; see also NEVILLE described for the elytra of Carabidae “Balken- 1975, 1993, 1998): lagen”(p. 447) consisting of fibrous chitin Fig. 1: Some beetles, whose elytra were studied (shown to scale). Abb. 1: Einige Käferarten, deren Elytren untersucht wurden (im richtigen Größenverhältnis zu- einander). A Pterostichus niger (Carabidae). B Cetonia aurata (Cetoniidae). C Anoplotrupes stercorosus (Scarabaeid- ae). D Tenebrio molitor (Tenebrionidae). E Chrysolina fastuosa (Chrysomelidae). F Ceutorhynchus palli- dactylus (Curculionidae). G Trigonopterus nasutus (Curculionidae). H Sitophilus granarius (Curculioni- dae). I Phyllobius betulinus (Curculionidae). On the architecture of beetle elytra 193 Entomologie heute 22 (2010) 194 THOMAS VAN DE KAMP & HARTMUT GREVEN Tab. 1: Thicknesses of the elytra and widths of cuticular layers, occurrence of plies and “balken”, size of the hemolymph space and capability of flight (indicated by symbols) of the beetle species examined. All measurements in µm. *Exocuticle of the ventral cuticle; **the small values refer to the elytral grooves, ***measurements above the haemolymph space. el = overall thickness of On the architecture of beetle elytra 195 layers (“fibrilläre Chitinschichten”; p. 447). were from the collection of the State Museum Later, DENNELL (1976; p. 162) distinguished of Natural History in Karlsruhe (SMNK). Most clearly between ”layers of bundles” (= balken; of them were stored in ethanol. see also DENNELL 1978) and “layers of For light microscopy small pieces of the elytra horizontally disposed fibres”. In the more were dehydrated and embedded in epoxy resin recent literature, especially studies focusing on (SPURR 1969). Semithin sections, approx. 0.5 the mechanical properties, only µm thick, were made with a Reichert OM U3 pseudorthogonal cuticles without further microtome using glass knifes and stained differentiation are mentioned and occasionally with toluidin- blue-borax. Photos were edited the cuticle structure is insufficiently described with Adobe Photoshop CS3. Measurements (e.g. CHEN & FAN 2004; YANG et al. 2010 and (see Table 1) were taken from sections of a references therein). single specimen of each species. Apart from the histological studies mentioned For scanning electron microscopy (SEM) above and a histological study comparing the dried elytra were broken vertically with two thickness of the various lamellae and the fine forceps. The pieces were glued to stubs, proportions of exo - and endocuticle in the sputtered with gold and examined under a elytra of 36 beetle species (KRZELJ 1969), Zeiss LEO 1430VP scanning electron detailed comparative studies of beetle elytra microscope. In some preparations the angle using transmission and scanning electron between two successive plies or “balken” microscopy, are largely missing. could be estimated by viewing the samples